Investigation structural settlement by Ground Penetrating Radar (Case study)

Electromagnetic wave is transferred by the GPR (ground penetrating radar), and A geotechnical application may benefit from this non-destructive test. This study is proposed to estimate the type and soil problem location that causes differential settlement of a structure (pumping station) by GPR surveying. The survey is achieved before and after the treatment by cement injection method to identify the locations that took cement injections as a full injection, partial or not at all using two types of antennas (160,450) MHz. The study also will estimate the thickness of the foundation by GPR and comparing it with actually executed. The results showed the creeping soil has occurred in some parts of the soil under the foundation, and after soil treatment, most of these parts were taken injection, and others did not. Also, it was found the relatively high accuracy of GPR for detecting the thickness of the raft foundation.


Introduction
Due to its non-destructiveness and the need to better understand near-surface conditions, ground penetrating radar (GPR) has become a commonly used geophysical instrument. As a non-destructive technique, GPR makes use of microwave radiation (UHF/VHF Frequencies) to find reflected signals from underground objects. Building materials, roads, bridges, and other subsurface infrastructure may all be studied using ground penetrating radar, which is a type of electromagnetic wave used primarily to probe the earth's low-depth subsurface. The GPR instrument detects soil or rock deformation by comparing the energy reflected off the body to the energy emitted, with the depth being estimated by the time it takes for the wave to travel. Antenna frequencies in GPR range from 80 MHz to 1000 MHz (each with a different number of channels). The accuracy and depth required dictate which antenna is best [2,3]. The GPR has been used to inspect many geotechnical engineering applications, including solid rock depth, buried foundations, and a variety of problematic conditions that may occur below the ground, such as cavity formation, discontinuities in layers, soil collapse, landfill creeping, and other uses [4]. In previous studies, Bakir H. B (2008) [5] detected weak zones at a proposed dam site by using the ground penetration GPR technique with the electrical resistivity (ER) technique, which is located in Koya city-Sulaimanya, north of Iraq. In this study, a 100 MHz unshielded antenna was used with the portable control unit. Many anomalous like limestone rock blocks and a large zone of cavities in addition to groundwater table have been detected. The main aims of this study are estimation the type and locate soil problem that causes differential settlement of the pumping station structure by GPR and surveying the soil before and after the soil treatment (cement injection method) to identify the locations that took cement injections or not and estimation the thickness of foundation by GPR and comparing it with that found in the actual design using two types of antennas (160, and 450 MHz).  [6] studied the ability to use the GPR with 250 MHz and 500 MHz antennas (middle frequencies) to explore the reinforced bar of concrete to show the number of steel bars and their configurations in the concrete constructed in the hidden mensuration. Another study by Karim and Al-Dami (2012b) [7] was achieved to simulate GPR data got by 250 and 500 MHz antennas for low deep inspected by discovering different underground bodies.

Theoretical Background
To understand the procedure, you must first understand the fundamentals of electromagnetic wave transmission processes. To investigate the underground, GPR systems use extremely high rates of electromagnetic energy. The dielectric characteristics of the substance at a high frequency determine the transmission of the radar signal [8].
The radar waves' speed and attenuation are measured using GPR techniques. These can be used to determine the dielectric value or relative permittivity of geological layers at very high frequencies [9]. The travel periods of reflected waves from subsurface boundaries are documented as their reach at the surface in this technique, and the depth, D, to boundary lines is calculated using [10]: Thickness (or Depth) = velocity × (time/2) D=TV/2 (1) Where: D: the reflector's depth. V: the speed at which the radar signal pulses through the subterranean bodies. T: the time it takes to get to the reflector in both directions.
The speeds of spread radar signal are related to the dielectric properties and relative permittivity (or relative dielectric constant) (ϵr) [9].
Where (= / 0 ) is the relation of the dielectric permittivity in medium to the dielectric in free space (=8.85×10-12F/m), µ (=µ /µ 0 ) is the relative permeability of the medium which is unity for most earth soils and rocks, and c = 3×108 m/s (=0.3 m/ns) is the speed of EM waves in free space. Since µ is close to unity for most rock materials, radar speed is mainly controlled by the dielectric constant of the medium as µ ≈ 1 [9, 10]: The thickness of pavement is found by the wave period of the device, as to equations (1) and (4): Where: h: The thickness of a layer is measured in meters. c: free-space light speed (≈ 300 m/s) t: time in both ways, in seconds. ϵ(r): constant of dielectric property of the substance material as shown in Table 1.

Location and Geology of the Site Study
The site of this study represents the pumping station which is located in the southeast of Baghdad-Iraq; the site has dimensions (45 * 90) square meters. The soil has been indicated by four boreholes (BH1, BH2, BH3, BH4) at the site study with depth each one of 30 m. represents the soil section in these boreholes and the geotechnical properties of the tested soil.
Rotary Auger drilling was used to carrying out the field investigation with core barrel for continuous coring and thin wall tube samplers according to standards of the American Society for testing and materials (ASTM D6151-08). The visual classification and grain size analysis results indicate that the soil ranged in classification and was composed mostly of two main layers clayey soil layer with color dark brown till about 13.0 m depth can be classified generally as low plasticity clay (CL) then follow about (13.0 to 30.0) m by sandy to silty soil with dark gray color soil can be classified generally as silty sand (SM). the chloride (0.03%-0.5%).

Instrument and Software Used
The version of the GPR device that was used in this study (MALÅ GX) with a system containing different parts (monitor, antenna, other tools) as seen in ' Figure 1', Ground vision Program was used in this study. Many filters were used within this program, filtering of radar data is used as an attempt to remove the unwanted signals (noise), and correcting the position of reflectors on the radar record, filtering such as (Band Pass, DC removal, Automatic gain control, Running Mean Trace, Background Removal, Subtract Mean Trace). One can use more than one filter for the same radargram to show the results more clearly as needed.

The first scanning works by a GPR device (before cement injection process)
The number of the surveyed paths inside the station is 118 paths*2 antennas =236 paths, and 19 paths*2 antennas = 38 paths outside the station and the distance between the path and other was 1.0-3.0 m as a grid lines as illustrated in ' Figure 3'. To detect the cause of the soil problem, the antennas type 160 MHz GX and 450 MHz GX were used along the aforementioned paths.

Improvement the soil by cement injection
To improve the properties of the soil under the foundation of the structure in which the settlement problem occurred.
The cement liquid was injected by pumps through two sets of tubes: 1) Pipes 3-inch vertical tubes with different depths (6, 14, and 18 m) in a 3-row shape around the structure to be doing in the form of a concrete wall in the soil around the structure that prevents the exit of any injected later with a group of half-inch diameter tubes to let water out of the soil during the injection. 2) Pipes of diameter 3 inches inclined at an angle of 45 degrees, starting from the surface around the structure and ending under the foundations of the building, with length pipes of 18 to 24 meters. More than 600 tons of sulfate resistant cement (SPC) was used in the injection process as seen in ' Figure 2'.